Graphical perception and cognition

MACS 40700
University of Chicago

April 17, 2017

What makes a graph more accurate?

A graphical form that involves elementary perceptual tasks that lead to more accurate judgments than another graphical form (with the same quantitiative information) will result in better organization and increase the chances of a correct perception of patterns and behavior.

Figure 6.12 from The Functional Art

Bar chart

Pie chart

Statistical maps with color

Scatterplot

Curvature

Curvature

Experimental design

  • \(N = 55\)
  • Experiment 1 - asked to make assessments of length and position along a common scale
  • Experiment 2 - asked to make judgments of position and angle (pie vs bar chart)
  • Make visual assessments of what percentage one value was of a larger value

Picking a task

A screenshot of data. Source: The Functional Art

A proportional symbol map. Source: The Functional Art

Chloropeth map. Source: The Functional Art

Dot chart. Source: The Functional Art

Scatterplot. Source: The Functional Art

Slopegraph. Source: The Functional Art

What we learn

  • Use theory and experiments to develop rules for data visualization
  • Different perceptual tasks are easier or harder to decode
  • Pick the highest-level encoding task possible given the data structure/purpose of the visualization

Amazon Mechanical Turk

Transparency of gridlines

In defense of pie charts

  • Which is easier to distinguish: line length or area, angle, and arc length?
  • Psychophysical theory of perception:

    \[\text{Subjective area} = \text{Area}^.86\]

  • Focus more on the subarea
  • What is the point of the chart?

In defense of pie charts

A B C D
10 20 40 30

Takeaways

  • For pure magnitude identification, bar charts are superior
  • For comparing percentages, either chart is acceptable
  • To compare combinations of groupings, pie charts are slightly superior
  • Tables are only useful if you want to report exact percentages

Design a data visualization experiment